The existence of discrete propagation modes in optical fibers can be both readily explained and observed The theory is derived from the application of boundary conditions to Maxwell's equations for light within an optical waveguide. Important properties of such waveguides include their all-dielectric nature, as well as the fact that they are nearly transparent to a limited band of optical frequencies. Beyond the plethora of theoretical analyses, fiber modes are often ignored, or only considered in that they affect the communication of information. Specifically, intra- and intermodal dispersion have been quantified in terms of the limitations they impose on fiber bandwidth and signal integrity. Also, a phenomenon known as modal noise has been identified as a major hindrance to AM signal transmission in optical fibers.
In addition to communications, optical fibers have received much interest as sensors of a host of mechanical, electrical and chemical parameters. This has led to the proposal and development of many new fiber types and applications with a view towards exploiting them for use in fiber sensors. In particular, consideration of the distance after the injection of light which is necessary to insure equilibrium mode power distribution is seen as applicable to certain sensor types. The mechanisms for coupling of power between modes have also come into focus due to their relevance to fiber sensors.
In a prior art fiber optic sensor, a sensor is coupled to a piece of mechanical equipment. The sensor is an optical waveguide such as a fiber optic cable. The light transmission characteristic of the optical waveguide is altered by the vibration or mechanical force imparted to it from the equipment to which it is coupled.
Two inter-related sensing methods which have their basis in mode phenomena have received some notoriety. The first involves the operation of a fiber such that only a few low order modes are allowed to propagate. Interferometry between these modes is performed to infer disturbances along the length of the fiber. The second method is derived from a multimode fiber output pattern which changes with fiber perturbations.
Optical fibers are often characterized by the amount of signal distortion they display over a given length. This distortion, referred to as dispersion, primarily results in pulse broadening, which thus introduces limits on the fiber bandwidth. Three major mechanisms are identified as contributing to the total dispersion: chromatic dispersion; wavelength dispersion and intermodal dispersion.
The first two types of dispersion are considered intramodal; that is, they affect the light propagating within a particular mode. Chromatic dispersion (alternatively called material dispersion) comes about because glass is a dispersive medium; the different wavelengths of light emitted by an optical source travel at different speeds in the fiber. Depending on the length of the fiber, the different colors will be separated in time, leading to pulse spreading.
On the other hand, waveguide dispersion results because the modal propagation constant .beta. is a function of the core radius-wavelength ratio. This effect can usually be ignored for multimode fibers, but can dominate in single mode fibers operating near the zero-chromatic-dispersion wavelength, especially when the fiber demonstrates any significant degree of birefringence.
Of more interest is the so-called intermodal dispersion illustrated in the step index ray trace diagram of FIG. 1. Higher order modes, propagating at steeper angles than the lower order modes, actually travel a longer distance in the fiber. Alternatively, the axial group velocity of these high order modes is slower than the velocity of the low order modes. This again leads to a broadening of an input pulse as a function of propagation distance.
In addition to fiber length, intermodal dispersion also depends on the numerical aperture of the fiber, itself a function of the core and cladding indices n.sub.1 and n.sub.2, the core radius, a, and the light wavelength .lambda.. Starting with a simple relation between the group delay t.sub.g, the length L, the propagation constant and the light frequency : ##EQU1## an expression can be developed for the modal delay due to both wavelength and waveguide parameters. For a single wavelength, the maximum time delay between modes can be very nearly approximated as: ##EQU2## where c is the speed of light and V is the normalized frequency. By only considering the propagation time difference between the highest and lower order modes, the simple ray trace model yields a reasonable approximation for the intermodal delay in terms of the index difference .DELTA. : ##EQU3##
In step index multimode fibers, the intermodal dispersion generally predominates the information-carrying capacity (expressed in terms of the bandwidth-distance product). One solution has been to manipulate the waveguide parameters and extend the wavelength so that only a single mode propagates, thereby eliminating intermodal delays. However, the small core sizes and the expense of long wavelength sources required for single mode operation are often prohibitive.
An alternate solution, though not as effective, appeared with the introduction of graded index fibers. Here, the index profile is arranged such that light travelling near the core-cladding boundary actually has a higher velocity than the light near the fiber axis. Rays travelling in a graded index fiber follow a spiraling trajectory where the high order modes round the corners more quickly than in the step index case. This leads to a significantly lower modal time delay and, therefore, increases the bandwidth-distance product.
In most cases, light propagating in optical fibers is eventually converted to a more usable electric signal by means of a photodetector. In addition to the noise created by the detector itself, unwanted electrical fluctuations due solely to changes in the mode propagation constants can be identified. This phenomenon, termed "modal noise", was first isolated and thoroughly investigated by Epworth at the Standard Telecommunications Laboratory in England in 1978.
In order to understand how modal noise occurs, it is first necessary to understand the nature of an optical fiber output pattern, known as a "speckle pattern". As modes travel in a fiber 2, they can be considered as rays, each with its own planar wavefronts, as pictured in FIG. 2. Provided that these modes are coherent along the entire fiber length, they will create an interference pattern at the end face 4. It is important to realize that any two modes will create a type of fringe pattern, and that if either changes its propagation characteristics, the entire fringe pattern will shift. Furthermore, on the end face, the fringe, or speckle, will measure about half the fringe spacing, approximately .lambda./2(NA), where NA is the fiber numerical aperture.
In practice, many more than two modes usually propagate, and the output pattern is generally imaged some distance from the fiber end. The result is a complex collection of light and dark spots of varying shape, intensity and polarization, arising from the multiple superposition of many wavefronts. These speckles are all interlinked and their spatial intensity distribution depends on the exact propagation features of the modes. These features include anything which affects .beta. ,such as the input wavelength, the injected power distribution, and the local refractive index and waveguide geometry.
Theoretically, the number of speckles which add together can be approximated by: ##EQU4## Although this expression is nearly identical to that for the number of modes in a step index fiber, it is important to bear in mind that the speckles are not the modes, but rather, the result of interference between the modes. The value in this relation is that it gives guidelines for increasing or decreasing the number of speckles, as desired for certain applications.
In order for interferences to occur, it was assumed in the foregoing discussion that the modes were mutually coherent. In a laboratory where only gas lasers are used, this is easily achieved since the coherence length of an average gas laser is often hundreds of meters. In the standard communications-grade semiconductor laser, the coherence length could exceed tens of meters. When the path length difference between two modes exceeds the coherence length, these modes will no longer interfere, but will merely add in their intensity distribution. This means that the speckle pattern will tend to defocus with increasing length along a given fiber. For most laser sources and fibers, however, total blurring of the speckle pattern is unlikely since neighboring modes remain coherent for relatively long distances. Note, in contrast, that LEDs are nearly incoherent sources to begin with; fibers illuminated by LEDs generally show no speckle. Rather, their output pattern is a smooth Gaussian-like intensity distribution.
Several mechanisms can create modal noise. The mechanisms which are most important to sensor work are changes in local fiber geometry, which in turn, alter propagation constants. These changes result from any small perturbations of the fibers, thus dimension) variations. All of these effects lead to amplitude modulation of the detected light. This would clearly be considered as noise in the context of analog optical signals.
Three major cures have been identified for modal noise, all affecting rather different domains of optical communications. The first most obvious solution is to use single mode fibers. This eliminates speckle altogether since there are not two or more modes to interfere with one another. Single mode operation is also attractive from the point of view that increased bandwidth capabilities and/or longer distances can be spanned. Some drawbacks occur however in that a single mode fiber is more difficult to handle owing to its small size, is more expensive and necessitates the use of laser diodes--themselves expensive and requiring complex driving circuitry. Also, single mode fibers usually show some degree of birefringence. When light passes any place in the fiber link which is polarization selective, what is known as polarization, modal noise occurs.
Another approach to reducing modal noise is to transmit purely digital signals. Thus, if high and low levels are set beyond the maximum amplitude of the noise signal, the unwanted AM component is lost. This has been done in practice for other reasons besides just avoiding modal noise. Still another scheme is to simply use a highly multimode laser; that is, one which shares its output power among many cavity modes. In addition to being unrelated to one another, the laser modes produce spectral lines of reduced coherence length. Other techniques basically attempt to cause a single mode laser to behave as multimode, for instance, by encouraging frequency dithering through the application of a radio frequency bias to the laser.
It is usually true that the very phenomena which the communications engineer finds most troublesome are the same phenomena which are exploited by those interested in developing fiber optic sensors. The classic case in point involves microbends occurring in fiber cables, both residually and after it has been laid. Microbending causes signal loss or attenuation and thereby limits the reliable transmission length of a fiber.
Strictly speaking, bend loss sensors are not considered modal sensors since the phenomena they measure appear as amplitude changes in the detected signal. We have seen, however, that periodic fiber deformers can actually be viewed as causing mode redistribution and since some of the modes escape the fiber, they lead to intensity loss. This differs from the sensors conventionally classified as modal domain, in that the latter generally do not depend on an average power loss in order to derive a sensor signal
One of the first truly mode-dependent sensors was developed at the U.S. Naval Research Laboratory around 1978. At the Naval Laboratory, the phase difference between any two modes in a step index fiber due to acoustically induced pressure variations was derived. Around the same time the dual mode sensor was being developed, researchers in England were investigating a similar device based on multimode propagation. The device called the "Fiberdyne" operates on the principle that optical fibers exhibit the property of self-homodyning, that is, that disturbances in the fiber induce a modulation of the multiple interference speckle pattern.
The Fiberdyne method has been used both in sensor configurations and as a transmission technique. In the first case, fiber carrying coherent light is usually subjected to some mechanical disturbance to be measured, such as an acoustic field. When the output is imaged onto a photodetector, an AM component in the output signal results. This is explained, as before, by noting that no system is entirely free from points of speckle selective loss.
From the above discussion, it should be apparent that Fiberdyne sensors are best suited to measuring time-varying quantities, since it is changes in the speckle pattern that causes changes in the detected amplitude. Furthermore, low frequency variations, while possible to detect, present difficulty due to deep signal fades. These result from environmental condition changes which cause small fiber expansions or contractions, or worse yet, which affect the light source operating conditions. The modal distribution is in turn rearranged, albeit, only slightly, resulting in a moving interference pattern. However, these fades can be at least partially overcome by forcing them to go through their full cycle at twice the expected data frequency or higher. The resulting signal is sampled and low pass filtered to obtain the signal of interest.
Specific sensor applications based on this method have been considered. The first to appear consisted of a fiber wrapped around a piezoelectric cylinder. When a voltage was applied, the cylinder and thus the fiber deformed; the motion was electrically detected at a photodiode and acoustic disturbance was monitored.
Another device was constructed for the purpose of measuring fluid flow. A fiber carrying coherent light was placed within a copper tube through which the fluid ran. The inevitable turbulence in the flow was detectable by monitoring the speckle output. It was discovered that the turbulence frequency was related to the flow velocity, giving the device the characteristics of a reasonably, accurate flow meter.
Current sensing has also been performed using the Fiberdyne technique. A metal coated optical fiber was placed in a permanent magnetic field. When an a.c. current was passed through the metal coating, the fiber in the magnetic field deformed in proportion to the current amplitude. By spatially filtering the speckle pattern and detecting with an amplifier tuned to the a.c. frequency, a repeatable, logarithmic correlation with the current level was obtained
More recently, the Fiberdyne method has been applied to the analysis of vibrations in composite material structures. Optical fibers are attached to bars or panels and the structures are set in motion with some forcing function. With a knowledge of both the fiber layout on the panel and the constraints on the panel, the bending modes of the panel can be directly obtained from the spectrum of the light detector output. Again, spatially filtering the speckle pattern and electronically filtering the detector signal can significantly increase the signal to noise ratio.
Two other mode-related phenomena have also been considered. The first is a method for measuring the bandwidth of optical fibers based on their speckle patterns. A frequency correlation function relating the spatial distributions of speckles at two deferent optical wavelengths with the fiber bandwidth has been defined. The second is mode division multiplexing (MDM). Under this phenomena two different tubular modes in conventional graded index multimode fiber are launched and detected. MDM was originally conceived as an alternate or addition to frequency and wavelength division multiplexing.
U.S. Pat. No. 4,191,470 (Butter) relates to a laser-fiber optic interferometric strain gauge. A laser provides an input into two single-mode optical fibers. Interference takes place between the outputs of the two fibers rather than within a single multimode fiber. U.S. Pat. No. 4,525,626 (Kush et al) is directed to a fiber optic vibration modal sensor which makes use of a single multimode optical fiber positioned within or on a structure for which vibration measurements are to be detected. U.S. Pat. No. 4,408,495 (Couch et al) is another example of a fiber optic system for measuring mechanical motion or vibration of a body. This patent appears to be concerned with creating an optical waveguide that is bent beyond the critical angle at which the light directed through the waveguide is substantially, totally, internally reflected along the waveguide. U.S. Pat. No. 4,420,251 (James et al) is another example of a prior art optical deformation sensor. U.S. Pat. Nos. 4,421,979 (Asawa et al) and 4,477,725 (Asawa et al) are directed to optical fiber sensors which employ micro-bending for remote measurement of forces such as stress at several locations along a pre-determined length. U.S. Pat. No. 4,269,506 (Johnson et al) is of interest in that it discusses the influence of physical parameters on the length of a path of an electrically stretchable optical fiber. Finally, U.S. Pat. Nos. 4,295,738 (Meltz et al) and 4,342,907 (Macedo et al) are examples of optical fiber sensing devices.
Despite these advances, there is still a need for a highly reliable modal domain optical fiber sensor for detecting parameters such as strain. The present invention is directed toward filling that need.